The present invention relates to a reducing melt borosilicate glass which transmits comparatively more UV radiation and has improved water-resistance properties.
Glass with comparatively high UV transmission properties has numerous applications, e.g. in EPROM windows and in UV glasses, in photomultiplier windows and windows of spectrometer as well as in a protective tube of a UV lamp in a UV reactor. For example, polluted water or chemical process waste water is prepared for further processing, given germicidal treatments and/or disposed of in UV reactors with the assistance of UV radiation, particularly at 254 nm wavelength. A UV radiation protective glass tube and also an EPROM glass thus must have the highest possible UV transmittance and also at the short UV wavelengths such as 254 nm. Furthermore they must have good water-resistance, since they are exposed to the action of aqueous solutions over a very long time and eventually at elevated temperatures. Quartz glass is especially suitable in itself as a UV transmitting glass, however because of its comparatively high price and difficult workability it is Used only in those exceptional cases in which particularly good hydrolytic properties are required. A further disadvantage of quartz is that it is only poorly fusible with ceramic substrates (e.g. Al.sub.2 O.sub.3), Ni-Fe-Co alloy or with molybdenum because of the too low thermal expansion coefficients.
UV-transmitting glasses have already been developed, which however only partially fulfill the requirements for UV transmittance and water-resistance or hydrolytic properties. Borosilicate glasses are particularly useful as UV-transmitting glasses.
Japanese Patent Application 60-77 144 describes a glass with a composition (in % by weight on an oxide basis) of 56 to 70% SiO.sub.2, 16 to 35% B.sub.2 O.sub.3, 4.7 to 13% Na.sub.2 O and 3 to 7% Al.sub.2 O.sub.3,which has a thermal expansion coefficient of 3.8 to 5.8.times.10.sup.-6 K.sup.-1 in a temperature range of from 20.degree. C. to 300.degree. C.
Japanese Patent Application 60-21 830 describes a borosilicate glass with a composition (in % by weight) of 60 to 70% SiO.sub.2, 4 to 8% Al.sub.2 O.sub.3,18 to 25% B.sub.2 O.sub.3, 6 to 11% Li.sub.2 O+Na.sub.2 O+K.sub.2 O, 0 to 4% alkaline earth oxides and zinc oxide, and 0 to 3% fluorine. This glass has a thermal expansion coefficient of 5.0 to 5.8.times.10.sup.-6 K.sup.-1 in a temperature range of from 20.degree. C. to 300.degree. C.
Another UV-transmitting glass that provides comparatively high transmission of UV radiation is described in German Patent Application DE-OS 38 26 586. This glass has a composition (in % by weight) of 58 to 62% SiO.sub.2, 15 to 18% B.sub.2 O.sub.3,11.4 to 14.5% Al.sub.2 O.sub.3, 1 to 2.5% Li.sub.2 O, 5.5 to 6.5% Na.sub.2 O, 0 to 2% K.sub.2 O and 0 to 0.6% chlorine. The thermal expansion coefficient .alpha. of this glass is from 5.6 to 6.2.times.10.sup.-6 K.sup.-1 in a temperature range of from 0.degree. C. to 300.degree. C.
Another glass of similar composition is described in European Patent Application Nr.0 388 581. This glass has the following composition in Mol % of 60 to 70% SiO.sub.2, 16 to 20% B.sub.2 O.sub.3, 1 to 8% Al.sub.2 O.sub.3, 2.5 to 5% Na.sub.2 O, 0 to 3% K.sub.2 O and 1 to 6% Li.sub.2 O. The thermal expansion coefficient .alpha. of this glass is from 4.6 to 5.2.times.10.sup.-6 K.sup.-1 in a temperature range of from 0.degree. C. to 300.degree. C.
There is also a commercially available glass with the designation "8338" with the approximate composition in weight percent of 62% SiO.sub.2, 19.8% B.sub.2 O.sub.3, 6.5% Al.sub.2 O.sub.3, 7.4% Na.sub.2 O, 1.7% K.sub.2 O, 0.5% CaO, 1.4% BaO and 0.7% Fluorine with a thermal expansion coefficient .alpha..sub.20/300 of 5.5.times.10.sup.-6 K.sup.-1.
There is also a glass known with the designation "BU 54" with the approximate composition in weight percent of 64.8% SiO.sub.2, 20.2% B.sub.2 O.sub.3, 6.5% Al.sub.2 O.sub.3, 6.5% Na.sub.2 O, 1.8% K.sub.2 O and 0.1% chlorine. This glass with a water resistance according to ISO 719 of about 250 micrograms per gram glass power is however not suitable for many applications, e.g. in the tropics.
According to the current state of the art it is generally known that to make this kind of glass only trace amounts of UV-absorbing materials may be contained in it so that a high UV transmittance can be obtained. Particularly contamination by the Fe.sup.+3 cation is to be avoided. Iron compounds are present in many raw materials used in glass making as impurities. Since absolutely iron-free materials are too expensive, the presence of some iron compounds cannot be avoided because of costs. Generally one uses high purity materials in the art of UV-transmitting glasses so that the content of iron compounds in the finished glass is not more than 10 ppm iron oxide. Since particularly the iron (III) cation is a strong absorber of UV radiation, the iron (III) cation must be reduced to the nonabsorbing iron(II) cation in UV applications. This happens by using a suitable reducing agent, e.g. sugar, and is assisted, as needed, by melting under a nonoxidizing atmosphere, as described in EP 0 388 581 A, to prevent reoxidation of Fe.sup.+2 to Fe.sup.+3.
Melts made too strongly reducing can lead to formation of defect centers and, because of that, to unintended absorption in the visible spectrum with transmission loses into the UV range, or even to UV absorption, which naturally makes the glass unusable. In melts with reduction potentials which are too low in contrast the iron (III) cations are not reduced sufficiently to iron (II) cations, so that the remaining iron (III) cations do not allow the maximum possible UV transmission. As a result the redox potential of the melt must be adjusted and maintained according to the specific requirements of the individual application. Frequently however on melting or fusing already small fluctuations of the redox potential can lead to considerable fluctuations in the UV-transmission properties. This phenomenon has been characterized as an insufficient stability of the high UV transmittance of the glass. The highest stability for desired reduced valence of the polyvalent ions required for high UV transmittance (particularly Fe.sup.+2) is achieved in glasses with an optimumized glass structure of higher binding strength. The known glasses because of their nonoptimized glass structure have achieved only an insufficient stability of high UV transmittance.
Furthermore many of the known high UV transmitting glasses have water resistance values of &gt;100 micrograms Na.sub.2 O per gram of glass powder which is insufficient for use, e.g. as EPROM windows, in countries having a highly humid environment. For use in protective tubes in UV reactors the known glasses are generally inadequate, since this application requires a particular high water resistance of under 100 micrograms Na.sub.2 O per gram of glass powder. Multicomponent glasses, which can be used for this purpose and are very water-resistant are not known, but very expensive quartz materials can be used.